1. Field of the Invention
The present invention relates to a method and apparatus for polishing a workpiece, and more particularly, to a method and apparatus for polishing a workpiece such as a semiconductor wafer having a thin film formed thereon to a flat and mirror finished surface.
2. Description of the Related Art
As integration of semiconductor devices intensifies, the distance between the interconnects formed in the devices becomes narrower. When forming interconnects of a width not more than 0.5 μm through a photolithography process in particular, the depth of focus becomes shallower and the stepper requires a flatter imaging plane. One prevailing device for flattening or planarizing the surface of the semiconductor wafer is a polishing apparatus for performing chemical mechanical polishing (CMP).
As shown in FIG. 5, such polishing apparatus comprises: a polishing table 302 having a polishing cloth or polishing pad 300 on its upper surface for providing a polishing surface 301; a top ring 304 for holding a workpiece such as a semiconductor wafer W so that the surface to be polished confronts the polishing table 302. The apparatus is operated to polish the semiconductor wafer W by respectively rotating the polishing table 302 and the top ring 304, and by pressing the semiconductor wafer W against the polishing surface 301 by the top ring 304 at a predetermined pressure while supplying a polishing solution from a polishing solution supply nozzle 306 arranged above the polishing table 302 onto the polishing surface 301.
The polishing solution supplied from the polishing solution supply nozzle 306 comprises an alkaline solution containing suspended abrasive grains so that the semiconductor wafer W is flat and mirror polished through a composite process of a chemical polish process by the alkaline solution and a mechanical polish process by the abrasive grains. A fixed abrasive is also used lately, instead of the polishing cloth, in which abrasive grains made of a material such as cerium oxide (CeO2) are fixed by a binder.
As the polishing apparatus continually processes the substrates, polishing performance of the polishing surface 301 of the polishing cloth 300 is deteriorated. Therefore, in order to recover the polishing performance, a dresser 308 having a dressing member 310 at its lower surface is provided for dressing or resetting the polishing cloth 300 during periods such as for exchanging the semiconductor wafer W to be polished. In this dressing process, a dressing solution such as deionized water is supplied to the polishing surface 301 from the water supply nozzle 307, and the dresser 308 and the polishing table 302 are respectively rotated. The dressing member 310 of the dresser 308 is pressed against the polishing surface 301 of the polishing cloth 300 to remove the polishing solution and polishing debris remaining on the polishing surface 301 as well as to flatten and dress the polishing surface 301 for resetting the polishing surface 301. This dressing process is also called a conditioning process.
A process of polishing a semiconductor wafer and dressing the polishing surface using the above described polishing apparatus will be explained with reference to FIGS. 6A˜6D and FIG. 7. FIGS. 6A˜6D are schematic views showing the conventional polishing process, and FIG. 7 is a graph showing the rotation speeds of the polishing table during these processes. Table 2 also shows conditions of the process mentioned above.
TABLE 2POLISH WITHPOLISH WITHPOLISHINGDEIONIZEDSOLUTIONWATERDRESSINGPROSESS TIME60 seconds15 seconds17 secondsTOP RINGPOLISHINGPOLISHINGSTANDBYPOSITIONPOSITIONPOSITIONPOSITIONDRESSERSTANDBYSTANDBYDRESSINGPOSITIONPOSITIONPOSITIONPOSITIONPOLISHINGSUPPLYSTOPSTOPSOLUTIONDEIONIZEDSTOPSUPPLYSUPPLYWATERROTATION80 rpm80 rpm40 rpmSPEED OFPOLISHINGTABLE
The semiconductor wafer to be processed (not shown) is placed on a pusher 312 which is arranged adjacent the polishing table 302. As shown in FIG. 6A, during the polishing process using polishing solution, the polishing table 302 and the top ring 304 are rotated independently and the polishing solution is supplied from the polishing solution supply nozzle 306 to the polishing surface 301. At this time, the polishing table 302 is rotated at a speed of 80 rpm, as shown in FIG. 7. The top ring 304 receives semiconductor wafer from the pusher 312 and presses the semiconductor wafer against the polishing surface 301 at a prescribed pressure for 60 seconds to polish the semiconductor wafer.
After finishing polishing using polishing solution, water polishing using deionized water is performed as shown in FIG. 6B. In this process, the polishing table 302 and the top ring 304 are rotated at respective constant speeds and deionized water is supplied from the water supply nozzle 307 to the polishing surface 301. The polishing process using deionized water continues for 15 seconds, as shown in FIG. 7.
After finishing the polishing using deionized water, the polishing cloth 300 is dressed or reset by the dresser 308 for recovering the polishing performance of the polishing surface 301 (see FIG. 5), as shown in FIG. 6C. In the dressing process, the rotation speed of the polishing table 302 is lowered to 40 rpm, and the dressing member 310 of the dresser 308 is forced to slidingly contact with the polishing surface 301 while deionized water is supplied from the water supply nozzle 307 to the polishing surface 301. During this period, the top ring 304 is moved to a position above the pusher 312 and the polished semiconductor wafer is transferred to the pusher 312 from the top ring 304. After finishing the dressing process, deionized water supply is stopped, and the polishing solution is supplied from the polishing solution supply nozzle 306 to the polishing surface 301 to start a next polishing process, as shown in FIG. 6D.
In case of continually polishing the semiconductor wafers, at the time the next polishing process is started, the polishing cloth 300 (see FIG. 5) has just finished the dressing process so that the polishing surface 301 of the polishing cloth 300 is filled with the supplied dressing solution (deionized water). If the polishing solution for the next polishing process is supplied to the polishing surface 301 containing abundant dressing solution, the polishing solution is diluted by the dressing solution having a different composition, as shown in FIG. 6D, so that it is difficult to obtain an expected polishing rate even if the polishing is performed at the same conditions. Also, it is necessary to extend the polishing time to obtain a preferred polishing amount resulting in lowering of the throughput.
The present invention is accomplished to address the above mentioned problems and aimed to present a method and apparatus for polishing a workpiece which can polish the workpiece at a constant rate in a stable condition even when plural workpieces are continually polished.